The tiny Miami blue butterfly, reduced to a few hundred survivors on isolated islands off Key West, will be formally declared a federally endangered species on Friday.

The U.S. Fish and Wildlife Service, which last year evoked rarely used emergency rules to temporarily extend endangered protections for the butterfly, announced Thursday that the listing would become permanent.

“The Miami blue butterfly is on the very brink of extinction, and this finalized protection gives it a real shot at survival and recovery,” said Tierra Curry, a biologist at the Center for Biological Diversity in California. “The Endangered Species Act is 99 percent effective at preventing the extinction of the species it covers, so we do have a hope, under the safety net of the Act, of stopping the loss of this beautiful butterfly.”

Environmentalists, scientists and butterfly enthusiasts hope the status will focus more attention and research on the nickle-sized butterflies, distinctive for the vividly colored wings of males.

But there are complex challenges to reviving a population once common along coastlines from Daytona Beach to the Dry Tortugas but now isolated to the Marquesas Keys, where experts fear a single tropical storm could wipe it out.

Its decline has been blamed on an array of threats, from pesticide spraying and development to exotic iguanas and ants eating the plants the butterflies rely on for feeding and breeding. Climate change and hurricanes also may have contributed.

The Miami blue was considered extinct for a time after Hurricane Andrew in 1992 until the discovery of a colony of 50 living in Bahia Honda State Park in 1999, a colony that vanished in 2010. Others have since been seen in the Marquesas, a string of islands west of Key West. Scientists believe captive breeding and reintroductions are their best hope of recovery.